The invention relates generally to amplifiers operable over a wideband signal range, and more specifically relates to nonlinearity correction techniques for differential-pair amplifiers.
Many applications require amplifiers which are capable of operating over a wideband range of signals. For instance, digital storage oscilloscopes require amplifiers that are capable of receiving and amplifying signals over a very large range of voltage levels. A frequently used amplifier in such applications is the differential-pair amplifier or its cascode-connected configuration. In circuits which need to operate over large input signal swings, a significant standing current is normally required to reduce the effect of base-emitter voltage variations in the transistor of the amplifier over the dynamic range of the device. The problem becomes severe when low-value emitter-degeneration resistors are used since power requirements can become excessive. A further difficulty arises in trying to optimally match the static power of the transistors in the differential pair with that of the load to provide minimum power change when the stage is driven.
One well-known method of reducing amplifier distortion is through the use of feedback techniques. In feedback amplifiers, the final output is sensed and fed back into the input so that linearity errors and thermal distortions are cancelled to a large degree. While feedback amplifiers having high precision may be designed, their use tends to be more restricted to lower frequencies. This is because at high frequency, phase shift within the amplifier tends to impair the stability when the feedback loop is closed. To preserve a stable phase margin, it is often necessary to reduce the open loop gain, thus causing deterioration of the overall performance.
If nonlinearities are attempted to be corrected by optimally matching the static power of the transistors in the differential pair with that of the loads to provide minimum power change when the stages are driven, this optimum setting often results in a low static operating voltage existing across the transistors. This low static operating voltage can cause f.sub.T degeneration, particularly when large collector currents are required in the differential pair. The f.sub.T parameter is a commonly used parameter in transistors, being a measurement of the frequency at which the common emitter current gain becomes unity. This is due to an inverse dependence of the effect of the base width with collector emitter voltage. Thus, the combination of the need to use large collector currents in a differential pair amplifier on a low static operating collector emitter voltage often results in degeneration of f.sub.T from the optimum.
When the cascode configuration is employed, it is not always possible to accurately thermally balance the differential pair. U.S. Pat. No. Re. 31,545 describes a feed-forward technique for first order correction of amplifier distortion through the use of a correction amplifier which is connected to the main amplifier to sense base-to-emitter distortion and develop an error signal which is injected to an output node to provide cancellation of distortion from the output of the main amplifier. FIG. 3(a) of U.S. Pat. No. Re. 31,545 ilustrates how the input is monitored. A certain change in the input signal causes a certain nonlinearity .DELTA.V.sub.be that is monitored by transistors 100, 102 and emitter-degeneration resistor 105. This causes an equal and opposite compensatory input such that the sum of the compensatory input and the normal input are added at the output current at resistors R88 to minimize nonlinearities. It is important to note that using the approach described in the reissue patent, high-frequency performance is minimized because it requires that transistors 100 and 102 have characteristics that are the same or better than transistors 70 and 72.
An additional problem with the technique described in U.S. Pat. No. Re. 31,545 is that it provides only first-order cancellation of nonlinearities. In some circumstances, it is necessary to provide a more precise form of correction of nonlinearities.